Bob McDavitt's ideas for sailing weather around the South pacific

14 August 2016

BOB's Blog

Issued 14 August 2016

Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the
patterned world.

For more stories about the work that SEA MERCY has been doing in Fiji since
Cyclone WINSTON see
Including the new mural at Makogai School, an article in Yachting World
Magazine, and the arrival of their new landing craft in the Lau group.

Also there is an interesting blog at . The
writer, David Straton on NIMROD spotted how
1) Some reefs are positioned in the wrong place on some electronic and paper
2) Google Earth's maps can help show some of these discrepancies.
3) Google Earth doesn't show all the reefs, suffers from lacunae.

To illustrate this last remark check the reefs north of Bligh water – the
stretch of sea between Viti and Vanua Levu in Fiji on your e or paper charts and
on Google earth. The comparison shows that Google earth only decided to show
detail around Charybdis Reef and Yadua Island, and the remainder is … lacunae.
Bligh would not have been impressed.

David's blog shows an illustration of this problem in Tonga AND adds a link to a
kmz file which adds 60 waypoints for missing/dangerous Tongan reefs to your
Google Earth (it's a Dropbox link and you should be able to download without
joining Dropbox if you wish by selecting "download only") see

The Tropics
During WWII the bombers based at Mariana Islands would fly bombing raids using
high Altitude B-29 aircraft. This was an interesting learning curve
meteorologically…. We didn't know much about Jet Streams until these pilots
encountered them. Also it was a surprise to the USA flying crew that the
cyclones in NW Pacific run at a frequency of one a week. These crew got little
sleep in-between missions and soon got into a déjà vu-itis confusing one storm
with another. To help the flying crew keep track, the meteorologists started
naming the storm using the phonetic alphabet (Alpha Bravo etc.). This wasn't the
first time storms were named… that started in Australia in the 1890s thanks to
Clement Wragge.

I raise this point because this week we have two cyclones in the NW Pacific
CONSON (08W) and CHANTHU (09W) both named by JMA … it would have been
interesting if two C-named cyclones were coexisting in the mid-1940s.
CONSON and CHANTHU tracks may be seen at

The rain maps for the past weeks, from , shows a
build-up of convective activity over NW Pacific and around the Solomon Islands.

SPCZ=South Pacific Convergence zone.
SPCZ is active this week and stretching from Solomon islands across Northern
Vanuatu and Southern Fiji to the Minerva area. A LOW is expected to form south
of French Polynesia by mid-week and its associated trough should bring variable
winds to Tahiti. This week's SPCZ may be seen on with rain
accumulation set to ten days.

STR= Sub-tropical Ridge
HIGH crossing Tasman Sea is expected to travel across South Island by mid-week
and then build to east of South Island and over Chatham Islands until
mid-next-week, maybe rising to 1040 hPa, making it a Big Fat High (BFH).
Squash Zone: As this HIGH crosses the Tasman Sea on Mon to Wednesday, there
should be a squash zone of enhanced trade winds in the Coral Sea/New Caledonia/
Southern Vanuatu to Fiji area on the north side of this HIGH.

Voyage Outlooks:
Tahiti to the west
The data is showing a mostly dry week around Tahiti and OK winds for departure
until at least Sat 20 local. Any voyage to the west will need to sail around at
least one passing trough which may contain squalls, but this week seems to be
free of any squash zone as far west as Tonga. Looks best to go along 18S.

Between NZ and the tropics
A Low is expected to form over northern NZ on Monday and deepen as it travels
east along 35S—so later this week there should be an intense squash zone between
this low and the BFH over Chathams. This squash zone is likely to toss a lot of
swell onto NE coast of NZ after Wednesday. These swells may have a long period,
so if they are OK and the SE/easterly winds along the NE coast of NZ (after
Tuesday) are OK then a departure from NZ to the north is OK.

See my yotpak at for terms used.
See my website for information on tailor-made voyage forecasts
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Haniye Asghari said...
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Nancy Uhl said...

Bob, thank you so much for highlighting Sea Mercy's continued efforts in Fiji in your blog post. The correct web address for the Sea Mercy Winston Update Blog is :) Thank you again for all the support you have given the Sea Mercy Recovery Fleet during their efforts to help rebuild the remote islands of Fiji in the wake of Winston's destruction.

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